Woodward–Hoffmann rules
The Woodward–Hoffmann rules are a set of organic chemistry rules to predict the stereochemistry of pericyclic reactions. Pericyclic reactions are usually rearrangement reactions where the molecule is a ring (e.g. benzene ring).[1] They were written by Robert Burns Woodward (a chemistry professor at Harvard University) and Roald Hoffmann (a chemistry professor at Cornell University). Hoffmann was awarded the 1981 Nobel Prize in Chemistry for this work, shared with Kenichi Fukui who developed a similar model.[2] Woodward did not share the prize because he died two years before. Generally, the Nobel Prize is awarded only to living people. Woodward had already won a Nobel Prize in Chemistry for a different discovery.
A recent paper in the journal Nature describes how mechanical stress can be used to reshape chemical reaction pathways to lead to products that apparently violate Woodward–Hoffman rules.[3]
Woodward–Hoffmann Rules Media
The Woodward-Hoffmann rules in action: Thermolysis of 1 yields the (E,E) geometric isomer 2, whereas thermolysis of 3 yields the (E,Z) geometric isomer 4.
Some thermal and photochemical interconversions of substituted cyclobutenes and butadienes showing conrotatory (blue) and disrotatory (red) behavior.
Diagram showing the FMO picture of the WH rules for 4n electron electrocyclization under thermal control.
Diagram showing the FMO picture of the WH rules for 4n+2 electron electrocyclization under thermal control.
Diagram showing the FMO picture of the WH rules for 4n electron electrocyclization under photo control.
The transition state of a conrotatory closure has C2 symmetry, whereas the transition state of a disrotatory opening has mirror symmetry.
MOs of butadiene are shown with the element with which they are symmetric. They are antisymmetric with respect to the other.
Symmetry of Psi_2 of butadiene with respect to C_2 axis and sigma plane.
Symmetry of the MOs of cyclobutene with respect to the sigma plane and C_2 axis.
4 electron electrocyclization reaction correlation diagram with a conrotatory mechanism.
Related pages
- Woodward's rules for calculating UV absorptions
- Torquoselectivity
References
- ↑ The rules are based on molecular orbital symmetry. The rules cover electrocyclic reactions, cycloadditions (including cheletropic reactions), sigmatropic reactions, and group transfer reactions.
- ↑ "The Nobel Prize in Chemistry 1981". Nobel Media AB. Retrieved 2011-07-22.
- ↑ Biasing reaction pathways with mechanical force. Nature 2007 446:423-427 (See also the corresponding "News and Views" in the same issue of Nature)
Other websites
- Symmetry rules!, Sophie Wilkinson Chemical & Engineering News January 27, 2003 Volume 81, Number 04 CENEAR 81 04 pp. 59 ISSN 0009-2347 Article
- "SHMO calculator", Simple Huckel molecular orbital theory calculator Link Archived 2009-11-22 at the Wayback Machine